In the field of electrostatic copying employing fusable toners, various devices have been proposed to perform the fusing function, which devices rely upon radiant heat transfer, conductive heat transfer and even convective heat transfer. The present invention relates to apparatus relying upon conductive heat transfer and thus employs a hot roll fuser. The invention, however, is applicable to any appropriate roll fuser, heated or not. In hot roll fusers, desirably a backup roll is provided arranged so as to form a nip between the hot roll and backup roll. Preferably, the backup roll is relatively cooler than the hot roll.
With such an arrangement there is a tendency for toner to accumulate on the backup roll either from minor contact with the hot roll, from loose toner carried by air within the machine or from contact with a previously fixed copy which is passed through the roll pair for fixing an image on a reverse side. This toner must be cleaned from the backup roll for, although the toner may initially exist in a liquid state, toner accumulated on the backup roll can become sufficiently hard to emboss the carrier and may even lead to jamming of the fuser by preventing the carrier from passing through the nip.
The necessity for cleaning the backup roll is demonstrated by considering that, typically, the pressure between the hot roller and the backup roll is on the order of 130 to 140 lbs. per square inch and the hot roll temperature is in the range of 350.degree.-370.degree. (F.). Subjected to these conditions, toner on the backup roll can become hard enough, after being subjected to these conditions for a period of time, to actually emboss the carrier or paper passing through the roll pair. This is, of course, undesirable. Furthermore, the toner build-up on the backup roll, under the conditions of pressure and temperature normally encountered, can build up sufficiently to cause wrinkling of the carrier or paper and even jamming which necessarily results in terminating copier operation so that the jam can be removed.
Even before toner buildup on the backup roll becomes hard enough to cause embossing, sufficient heat is transferred to the backup roll to cause any toner located thereon to become tacky. Under these circumstances, the paper travelling through the paper path may tend to adhere to the backup roll which, of course, is also undesirable.
Prior art hot roll fusers have employed coated backup rolls coated to facilitate release. However, for a number of reasons, it would be desirable to employ a backup roll consisting of an uncoated conductor. One reason is cost; an uncoated roll is less expensive than a coated roll. Another reason is that electrostatic charging during image transfer to the paper tends to leave a residual charge on the paper. Desirably, this charge should be removed since it only inhibits proper paper flow. Clearly, a conductive backup roll will tend to "ground" the paper and drain off any residual charge thereon, whereas a coated backup roll will not perform this function, or will not perform it to the same extent.
A cleaning device for a coated backup roll comprises a scraper blade with a sharp leading edge composed of a plurality of individually flexible fingers for scraping toner from the backup roll is described in U.S. Pat. No. 3,794,417. Another cleaning arrangement employing a scraper blade for a backup roll in a hot roll fuser is shown in published patent application B579,116. The backup roll disclosed in both the aforementioned references comprises an aluminum cylinder with a thin surface coating such as polytetrafluoroethylene, aluminum oxide, chromium oxide or aluminum oxide imbedded within polytetrafluoroethylene.
Other arrangements for cleaning cylindrical surfaces in an electrostatic copier include that shown in U.S. Pat. No. 3,970,038 wherein a plurality of flexible fingers are supported for wiping contact, as opposed to scraping contact, with the fuser roll, or that shown in U.S. Pat. No. 3,940,282 wherein a pair of scraping blades are in contact with an image supporting surface.
It is an object of the present invention to provide a cleaning arrangement for an uncoated backup roller in a hot roll fuser. It is another object of the present invention to provide a cleaning arrangement which comprises a pair of scraper blades each having serrated or interrupted scraping edges in contact with the backup roll. It is a further object of the present invention to provide a cleaning arrangement of the foregoing type in which the serrations or interruptions of each scraper blade are offset with respect to the other such that the entire surface of the backup roll is scraped by either the first or the second scraper blade, or both. It is another object of the present invention to provide a cleaning arrangement for an uncoated backup roll which includes a flexible scraper blade, made flexible by the presence of serrations or interruptions in the blade surface which, at the same time, insures that all of the backup roll surface area is scraped by providing a second flexible scraper blade with serrations or interruptions in the surface of the scraper blade offset with respect to the first scraper blade.